Permanent magnet material powders having superior magnetic characteristics

ABSTRACT

The coercive force of cobalt-rare earth intermetallic compounds is greatly enchanced by treating the compounds in finely divided form with acid.

[Unite tee tet 51 Mar. 2%, 1972 [54] PERMANENT MAGNET MATERIAL WEWIDERS HAVING SUPIERHOR MAGNIETM QHARAQTERESTICS [72] 1nventor: Joseph .1. Becker, Schenectady, NY.

[73] Assignee: General Electric Company [22] 1F iled: Sept. 1141, 11970 [21] Appl. No.: 72,099

W Reliated US. Application Data [63] Continuation-impart of Ser. No. 730,577, May 20,

1968, Pat. No. 3,558,371.

[52] US. 61. ..l48/31.57, 75/0.5 AA, 75/170, 156/7 [51] um. C1. ..H01f1/06 [58] Fielld uiSearch ..148/31.57,100,101, 102,103, 148/105; 156/7; 75/0.5 AA, 170

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White Attorney chemes T. Watts, Paul A. Frank, Jane M. Binkowski, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [57] ABSTRACT The coercive force of cobalt-rare earth intermetallic compounds is greatly enchanced by treating the compounds in finely divided form with acid.

4 Claims, N0 Drawings PERMANENT MAGNET MATERIAL POWDERS HAVING SlJlPlEliillOlh MAGNETIC CHARACTESTICS This application is a continuation-in-part of copending application Ser. No. 730,577 filed on May 20, 1968, and now US. Pat. No. 3,558,371, entitled Permanent Magnet Material Powders Having Superior Magnetic Characteristics And Method" filed in the name of the present inventor and assigned to the same assignee.

The present invention relates generally to the art of making permanent magnets and is more particularly concerned with new magnetic material powders having unique characteristics and with a novei method for producing these powders.

it is generally recognized that the permanent magnet properties of bulk magnetic materials having large magnetocrystalline anisotropies can be enhanced by reducing them to powders. it is also common knowledge that such powders can be incorporated in bonding media to provide composite permanent magnets having properties substantially superior to those of the bulk source materials. These advantages are, however, offset to a substantial degree in some instances when the particle size reduction is accomplished by grinding. Thus, a comparatively low value of coercive force can substantially diminish the advantages to be gained by converting the bull: body to a powder and fabricating a composite finished article from the powder.

accordance with the present invention, the detrimental effects of grinding upon the magnetic characteristics of the cobait rare earth materials can be eliminated and the coercive force of mechanically reduced materials of this kind can be enhanced to a surprising extent. in essence, the method of this invention centers in the key step of chemically treating the finely divided, ground, magnetic material with an acid in a contact period sufficient to increase the coercive force of the powder significantly. Although the exact mechanism is not certain, it seems likely that the acid produces holes or channels in the particles in such a way that the form of the particle is changed with an attendant increase in coercive force.

This invention thus centers in the concept of subjecting these ground materials to an acid treatment to eliminate the degrading effects of the grinding upon their magnetic properties. The invention is also based upon my discovery that such treatment can result in surprisingly large increases in coercive force of these cobalt-rare earth materials.

The acids which are employed in this case are different in composition and apparently in effect from those disclosed and claimed in my copending patent application, Ser. No. 701,840, filed Jan. 31, 1968, and now US. Pat. No. 3,558,372, and assigned to the present assignee. The polishing effect leading to substantial increase in coercive force of the treated powder in accordance with my prior invention stands in contrast to the acid attack and erosion on the particles which leads to substantial increase in coercive force value in accordance with the process of my present invention.

As a general proposition, the chemical treatment is carried out according to this invention by contacting cobalt-rare earth magnetic material with an acid, the material in powder form preferably being immersed in the acid for the required period of contact time and then promptly removed and rinsed free from that acid in order to arrest the acid attack.

Time, temperature and acid concentration are interrelated factors in this method. l have found, however, that practical operating ranges of these variables are so broad that the necessity for precise control of the method can readily be avoided. Thus, except for threshold or marginal conditions of time, temperature and concentration, the operating conditions or a combination of these conditions is not critical to the success or failure of the method in terms of the products obtained. in accordance with my preference, the acid will be at room temperature but it may be at any temperature at which the acid is a liquid. At room temperature, i.e., about 25 C., the contact period generally ranges from about a few seconds to about 90 minutes. The specific contact period necessary to obtain maximum coercive force for a particular powder is determinable empirically and depends largely on particle size, the specific acid used and its concentration, and acid temperature. Generally, shorter contact periods are required when smaller particles are used or when more concentrated acids or acids substantially above room temperature are used. However, prolonged acid contact may lead to significantly diminishing magnet material yields, particularly if the treating acid temperature is substantially above room temperature.

Representative of the acids useful in the present invention are hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid and acetic acid. The concentration of the acid used may vary widely. Generally, the more dilute an acid, the longer is the contact time required to produce a maximum coercive force.

For experimental purposes, I have used cobalt-rare earth magnet material of particle. size ranging from less than 30 microns to l00 +170 mesh (between 88 and 147 microns) (U.S. standard screen sizes). Materials of this kind may, however, be treated according to this invention with the foregoing results when the particles are twice as large, but the maximum coercive force obtainable is lower because of the fact that coercive force generally varies inversely with particle size. Much finer particles may likewise be treated in accordance with this method but at the cost of smaller product yields because of the relatively larger proportion of each particle dissolved in the acid attack.

This invention method has been found in actual practice to be particularly beneficial in the treatment of cobalt-base permanent magnet materials which are Co -,R intermetallic compounds where R is a rare earth metal. Representative of these materials are Co Y, Co Sm and Co M (cerium-rich mischmetal).

While I do not intend any limitation on the claims, it is my belief and theory that the results obtained by applying this method to cobalt-rare earth permanent magnet materials can be explained on the basis that the acid produces holes and channels in the particles in such a way that the form of the particle is changed with an attendant increase in coercive force. The improvement in the coercive force characteristic of powders of such materials treated in accordance with the method is such as to give support to this theory and there does not appear to be any alternative explanation for this remarkable change in this key property of these materials. Moreover, there does not appear to be any chemical change produced in the magnet materials by the acid treatment of this method.

The following illustrative, but not limiting examples of operations embodying this invention which I have carried out are ofi'ered by way of further describing the present novel method to those skilled in the art:

EXAMPLE 1 An ingot of cobalt-Samarium (Co Sm) was ground with mortar and pestle. The resulting powder was screened and the fraction passing through a 325 mesh screen was selected for test. Substantially equal portions of this fine powder fraction were used in the preparation of four samples.

One sample was prepared by introducing a portion of the powder into a body of molten paraffin wax and cooling the wax in an aligning magnetic field of 21,000 oersteds until it was solidified.

Another portion of the powder was immersed in 2 percent nitric acid at room temperature, i.e., about 25 C. At the end of 30 seconds, the powder was removed from the acid, rinsed with water and with acetone, and permitted to dry in air. Additional portions of the powder were treated in the same manner except that they were immersed in the 2 percent nitric acid for 60 seconds and 90 seconds, respectively. Each of the treated, dried, powder portions was then mounted in paraffin as described above.

The coercive force of each sample was measured after magnetization in a field of 30,000 oersteds. The results are given in Table l.

TABLE 1 Coercive Sample Time, force,

No. Material Size Acid seconds oersteds l Co Sm. 325mesh 43 microns) None 7,500

2 Co Sm o 2%HNO; 30 8,450

3 00 8111". do a HNoam" 60 9,650

4 Co Sm ..d0 2% HNO3 90 11, 450

As illustrated in Table l, the coercive force increased signifimy As illustrated in Table IV, the coercive force of the cobaltcantly after relatively short periods of immersion in the nitric yttrium powder i i rea ed significantly after relatively short acid.

EXAMPLE 2 in this example, the procedure was the same as that disclosed in Example 1 except that the cobalt-Samarium was ground to a size less than 30 microns and 1 percent nitric acid was used. The results are given in Table II.

periods of immersion in the acid.

EXAMPLE 5 Cobalt-yttrium (Co l!) was used in this example. The procedure in this example was thesame as that disclosed in Example 1 except that the acids given in Table V were used,

TABLE II Coercive Time, force, Material Size Acid seconds oersteds Co Sm microns Nona 0 10,200 0053111 do HNO; 30 11,480 00 3111 do 1% HNO 11,580 0053111 ..do 1% HNO 11,690

As illustrated in Table II, the coercive force of. cobaltsarnarium powder as fine as less than 30 microns can be increased significantly by short periods of immersion in an acid as dilute as 1 percent nitric acid.

EXAMPLE 3 in this example the procedure was the same as that disclosed in Example 1 except that the ground cobalt-Samarium fraction passing a 250 mesh and returned on a 325 mesh screen was selected for test and 1 percent nitric acid was used. The results are given in Table III.

TABLE III Coercive Sample Time Force No. Material Size Acid (min.) (ocrstcds) 9 Co,Sm 250+325 None 0 1835 mesh 10 Cc,Sm 250+325 1% HNO, 2 4730 mesh As illustrated in Table ill, the coercive force of cobaltsamarium powder as coarse as the 250 +325 mesh was increased significantly afier a short immersion in an acid as dilute as 1 percent nitric acid.

EXAMPLE 4 Cobalt-yttrium (Co Y) was used in this example. The procedure was the same as that disclosed in Example 1 except that 1 percent nitric acid was used and the coercive force was measured after magnetization in a magnetic field of 21,000 oersteds.

The results are given in Table IV.

and the coercive force was measured after magnetization in a field of 21,000 oersteds.

Table V illustrates the number of different acids which may be used to increase the coercive force of the powder, and the approximate rates at which these particular acids increase :such force. Specifically, a comparison of Sample 22 of Table 5 ;V, where a 20 percent nitric acid was used, with Sample 13 of 5 Table IV, where a 1 percent nitric acid was used, shows how much faster the coercive force was raised by the more concentrated acid.

EXAMPLE 6 in this example cobalt-yttrium (Co Y) was used. The procedure in this example was the same as that disclosed in 5 Example 5 except that the ground fraction passing through a l 100 mesh screen and returned on a mesh screen was used for test and the acid was 2 percent nitric acid.

A sample of the powder as ground, i.e., not treated with acid, had a coercive force of 71 oersteds. A sample of the 70 powder was immersed in 2 percent nitric acid at room temperature for 6 minutes. At the end of this time its coercive force was measured as being 1,100 oersteds.

This data illustrates that even in this instance, where the initial particle size was so large, the coercive force was still significantly increased after a short period of immersion in the acid.

EXAMPLE 7 EXAMPLE 8 in this example cobalt-yttrium (Co Y) was used. The procedure used in this example was the same as that disclosed in Example 5 except that the ground fraction passing through a 250 mesh screen and returned on a 325 mesh screen was used for test and the acid was 2 percent nitric acid.

A sample of the powder as ground, i.e., not treated with acid, had a coercive force of 103 oersteds. This sample was placed under an optical microscope and a magnified picture of it was made. A sample of the powder was immersed in the 2. percent nitric acid at room temperature for 2 minutes. At the end of this time its coercive force was measured as being 1,860 oersteds. This sample was also placed under the optical microscope and a magnified picture of it was made. A comparison of the two pictures shows the untreated sample to consist of solid particles whereas in the acid treated sample the particles were porous to some extent and their surfaces were pitted.

Wherever percentages or proportions are stated in this specification, results are made to the volume basis rather than weight basis.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A permanent magnet material which is a C0,,R intermetallic compound, where R is a rare earth metal, in the form of a ground powder wherein the particles are porous and have pitted surfaces resulting in a permanent magnet with an increased coercive force.

2. The material of claim 1 wherein C0 R is selected from the group consisting of cobalt-yttrium, cobalt-Samarium and cobalt-mischmetal.

3. A permanent magnet material of claim 1.

4. A permanent magnet material of claim 2.

composed of the magnetized composed of the magnetized 

2. The material of claim 1 wherein Co5R is selected from the group consisting of cobalt-yttrium, cobalt-samarium and cobalt-mischmetal.
 3. A permanent magnet composed of the magnetized material of claim
 1. 4. A permanent magnet composed of the magnetized material of claim
 2. 